A HPLC method was developed to quantitate SR-2508, a new hypoxic cell radiosensitizer, in the urine and blood of patients. Data have been collected on a single patient entered into a study at NIH who received 2.0 g/m2 IV. SR-2508 disappeared with a distribution halftime of 15 min and a plasma elimination halftime of 6.6 hr. The renal clearance of the drug was 106 ml/min and the renal excretion of the parent compound was 110% of the total dose administered. The metabolism of 6-mercaptopurine was studied in vitro and in patients. 6-Mercaptopurine ribonucleoside was found in the blood and urine of patients. Also 6-mercaptopurine formed from 6-mercaptopurine by monkey liver microsomes.